In recent years, with downsizing and advancing functionality of electronics, printed wiring boards provided in electronics have highly densified wiring. In addition, with advancing functionality of electronic components mounted on printed wiring boards, the temperature of heat applied to such a printed wiring board is also rises. For example, a printed wiring board provided in electronics having strict demands for security, such as electronics mounted in an automobile, also has a higher demanded upper temperature limit.
Printed wiring boards may include, for example, multilayered printed wiring boards that is possible to be downsized and highly densified (for example, refer to Japanese Laid-open Patent Publication No. 04-208597).
A multilayered printed wiring board is equipped with a through hole electrically connecting wiring patterns of each layer with each other. Japanese Laid-open Patent Publication Nos. 2007-318035 and 2009-164358 are further examples of related art.
With the densification of wiring, through holes in printed wiring boards are also being downsized. Compared with larger through holes, smaller through holes are poor in strength of an electrically conductive material in the through holes (for example, copper plating).
In addition, with advancing performance of electronic components, operation temperature of printed wiring boards is also becoming higher. Such a printed wiring board expands and contracts in accordance with a change in an ambient temperature.
For example, an epoxy resin used for a printed wiring board has a greater coefficient of thermal expansion compared with an electrically conductive material formed in the through hole, so that thermal stress is applied to the electrically conductive material in the through hole when the printed wiring board expands and contracts.
As just described, through holes are in a situation that is prone to develop a crack associated with downsizing and a rise in temperature of a printed wiring board. It is difficult not to expand and contract a printed wiring board, which causes development of cracks, and intensification of structural strength of a through hole makes it difficult to downsize a printed wiring board.
Consequently, it is considered that avoidance of defects in electronics derived from a trouble in a printed wiring board is realistic action.